Photosensitive resins used for the manufacture of relief printing plates are preferably developable with water rather than organic solvent for various reasons such as ease of handling, health of workers who come in contact therewith, safety, and avoidance of environmental pollution. Printing plates employed for flexographic printing must be capable of printing on a variety of substrates, which vary widely in their composition and surface uniformity. Substrates employed for flexographic printing include metal foils, plastic films, kraft paper, corrugated board, laminated papers and boards, newsprint, and the like. In order to successfully print on this wide variety of substrates, one must use a variety of ink types and printing press conditions.
The ideal printing plate for use in flexographic printing applications would be one which is capable of withstanding the potentially deleterious effects of the agents which are likely to come in contact therewith, i.e., common ink solvents, developing media (e.g. water), radiation curable materials, and the like. The ideal printing plate will further show no tendency to crack when flexed, and will be soft enough to conform to irregular surfaces, yet durable enough to withstand the forces of the printing process. It is also desirable that the resin material employed for the preparation of printing plates resist cracking or degradation when exposed to ozone in ambient air.
Several types of photosensitive resin plates have been used for flexographic printing. For example, photosensitive resin plates prepared employing modified thermoplastic elastomeric rubber or rubber-like printing media have excellent mechanical properties. Unfortunately, however, such resin plates can be processed only in organic media (see, for example, U.S. Pat. Nos. 4,323,673, 4359,246 and 4,622,088).
More recently, solid flexographic printing plates which are developable in water have been described. See, for example, U.S. Pat. No. 4,956,252 (which describes the use of a core-shell microgel to produce a photosensitive resin). However, this patent requires that crosslinking of the core-shell microgel be specifically located inside the core, while the shell remains uncrosslinked. The patent also discusses the need for the existence of two glass transitions (T.sub.g) in order to identify the core-shell nature of the microgel. In this patent, "microgel" is specifically noted to be used in a way other than in its conventional sense. Instead, the term is specifically defined as referring to a particle having two domains--a crosslinked core and an aqueous processable non-crosslinked shell. The core is explicitly stated to have less than 10% crosslinking and the shell is explicitly described as consisting of an acid-modified copolymer.
U.S. Pat. No. 4,726,877 also discloses the use of a core-shell microgel, but requires greater than 10% crosslinking in the material and a single T.sub.g of greater than 25.degree. C. This patent also requires that an additional polymer binder be used in the preparation of the microgel.
European Patent Application No. 0 604 876 A1 also describes the use of a core-shell microgel, with monomeric dienes required for use in the core thereof. This publication does not require the use of a crosslinking agent for the preparation of the core-shell microgel. The resulting material is particularly susceptible to ozone degradation.
U.S. Pat. No. 5,075,192 is another recent patent which requires a core-shell microgel. This patent, however, specifically requires the use of allyl methacrylate as a crosslinking agent to provide grafting between the core and shell.
U.S. Pat. No. 4,894,315 describes a core-shell microgel blended with a linear copolymer composed of elastomeric and thermoplastic domains. In this patent, a crosslinked elastomeric core and a non-crosslinked thermoplastic shell is specifically required.
U.S. Pat. Nos. 2,893,868, 4,275,142 and Japanese Kokai Publication 61-22339 each disclose water developable photosensitive resins useful for flexographic printing. The compositions disclosed in the U.S. patents have poor water resistance, and the compositions disclosed in the Japanese Publication have poor mechanical properties after curing. All three of these documents require the use of a core-shell material that is a copolymer of at least four monomers, one of which is a monomeric diene. This copolymeric material is difficult to produce in a consistent manner and is particularly susceptible to ozone degradation.
U.S. Pat. Nos. 5,348,844 and 5,073,477 describe flexographic printing plates using a material which requires an additional copolymer to act as a binder for the resin plate. Both of these patents require a material that is a copolymer of four different monomers, one of which is a monomeric diene. This copolymeric material is difficult to produce in a consistent manner.
U.S. Pat. No. 5,230,987 requires a T.sub.g of 60.degree. C.-125.degree. C. for the copolymer used as the binder, but does not require a crosslinking agent in the binder.
In all of the above-described materials, the lack of crosslinking in the binder produces a soluble polymer in the monomer mixtures. The presence of a soluble polymer will result in significant swell when exposed to printing inks. Such systems are therefore not acceptable for use in printing applications where more than one type of ink may be used.
U.S. Pat. No. 5,238,783 describes a sea-and-island morphology, requiring the sea to be gelled by a gelling agent, and the islands to be composed of alkylene glycols or derivatives thereof. However, this material does not provide significant improvement in water processability and still requires a thermoplastic elastomeric block copolymer (such as SIS) as part of the binder.
PCT Publication No. 92/15628 uses a comb polymer material as the binder for the resin. This material is not sufficiently water dispersible to be suitable as a flexographic binder in a water processable printing plate.
PCT Publication No. 93/03423 uses a microgel, but requires a tertiary ammonium salt to be incorporated into the microgel, which must be reacted with an epoxy group to form the resin. This is an additional processing step complicating the manufacture of the binder and the formulation of the resin.
Each of the above-described systems, however, suffers from significant drawbacks that limit the broad usefulness of the resulting resins. What is still needed in the art, therefore, is a material useful for the production of flexographic resins which has improved physical and mechanical properties, is processable in an aqueous environment, and is easily manufactured.